Dopamine (DA) has been implicated as an important target in understanding a number of psychiatric and neurological disorders and is believed to play a central role in addiction. Research supports a role for DA in both motivational processes and reinforcement learning. The nature of the relationship between these two aspects of DA, however, is controversial. Further complicating matters, DA exhibits two distinct types of activity, phasic, burst-like firing and tonic pacemaker-like activity. These two types of activity may be differentially associated with motivation and reinforcement learning. At present, however, it is difficult to study the contribution of one type of activity independently of the other. In this proposal we will create two transgenic mouse lines that result in alterations in phasic DA activity leaving tonic activity unaffected. We will then behaviorally characterize these mice using test paradigms that dissociate motivation from reinforcement learning to assess the independent contribution of phasic DA activity to each. A computational model formalizing a hypothesized relationship between reinforcement learning and motivation and incorporating both phasic and tonic activity will be developed